If anybody is going to actually prove or disprove it, they need to have two experiments going on. One where the learner receives comprehensible input and one where the learner does not, with all else being equal. For the study, I would suggest using the same learner in both experiments. First show me that the learner could not acquire a language through incomprehensible input, and then show the same learner could acquire a different language through comprehensible input. For example, first the learner tries to acquire Japanese, and then Korean. They are both similarly structured languages and would make a good comparison. You could not use the same language for the second experiment as the first because you would prove nothing since the learner received all the exposure to the language in the first experiment, so you must use a different but similar language for the second experiment on the same learner. But you could not use Portuguese and Spanish because they are way too similar with vocabulary coming from the same Latin roots.
Anyway, enough of that. Nobody is going to do their SLA thesis based on my experimental guidelines! So let me tell you what I do think is important about comprehensible input.
The importance of comprehension is that it encourages the learner to pay attention and to continue the process of receiving input. In other words, they don't quit when they are enjoying what they are getting. It can be evidenced from the ALG World classes, that when learners get to level 3 and the comprehension drops, the students get frustrated and drop out. The props and drawings are taken away and learners go into shock.
Quotes from AJ:
At level three things get much worse at AUA. Other students had warned me about the “level 3 shock” but I was still surprised by it....
My rough guess is that the Level 3 input at AUA is only 35-50% comprehensible (to me and most new arrivals). Sometimes the comprehension level is much lower-- and I have no clue what’s going on. Input that is not comprehensible is wasted...
More from AJ:
Through levels 1 and 2 I was laser focused in class. I was not understanding everything...maybe 70-80%... but usually the major points of what was going on....
But when I hit level three my superb concentration and focus vanished. Suddenly I was drifting off in class... daydreaming.... looking at the cute girls in class... thinking about what to do when class was over. I became bored. Not that I didn’t try. I made heroic efforts to keep focused, but could not sustain them. I just reached level 4-- I understand more but am still often confused. My motivation has plummeted. I’m skipping class constantly.
So what happened... what changed? AUA’s classes were never 90%+ comprehensible, but I did OK in the first two levels. Why? Because in levels 1 and 2 the teachers were encouraged to use a wealth of drawings and props and charades and games-- in other words-- aids to comprehension.
At levels 3 and up, the teachers/managers inexplicably decided that comprehension aids were no longer desirable. No more games. No more drawings. No more charades.....
Faced with two stationary talking heads, my understanding plummeted. And, as in the TV experiments, so did my attention. Occasionally I’d get drawn back if I heard something I understood... but quickly tuned out again once confused.
So what makes comprehensible input important is the fact that you stay focused on the input and don't feel like giving up! That's why it is good to have. But is it really necessary? I say that for what is going on inside your head, it is not necessary.
First of all, for the most part, that which is done to create comprehensibility is not the language itself. Drawings, gestures, props, and everything else is a visual aid to comprehension, not a language aid. The communication can be made comprehensible, but still the actual language used is not yet learned. So comprehension does not equal acquisition. Comprehension can be quick, but acquisition takes time.
Second, Dr. Brown said that students should not think about the language. So if we are not thinking about the language but yet we can still acquire the language, then acquisition must happen regardless of the comprehension.
Basically, comprehension is something you do, but language acquisition is not something you do, rather it is something that happens to you. How does it happen to you? Through lots of input that you let into your head.
A language is not a randomly used set of words. All languages are used in patterned ways. These patterns are studied by linguists and then classified into grammar rules. Then, teachers teach these grammar rules to native students and to language learners. The rules help keep people using the same set of patterns in the language.
Through massive amounts of input, our brains will organize the patterns of the new language and create firm paths for us to access our new language. We can't do this consciously. By studying the language, learning the grammar, memorizing words, and so on, we create a habit of thinking about the language and we try to reproduce the language before we have enough exposure to it.
1,500 hours of study may create a person who is very skilled at reproducing the language. But 1,500 hours of natural language acquisition produces a person who thinks like a native in the language. 1500 hours of thinking vs. 1500 hours of not thinking.
The "thinking vs. not thinking" is the important difference in the speed of further and further acquisition. Thinking is a habit! How hard it is to break reinforced habits! The reason ALG World doesn't allow you to speak during the first 800 hours is because they don't want you to develop the habit of thinking. If you know only 500 words of Thai, and you go around practicing it, you're going to find yourself in situations where you need words that you don't know. Or you'll know some words that you think are correct and you'll think of a way to string together what you mean with them. The point is that you're creating a habit of thinking about how to say what you feel. You also end up saying things that you don't know if it is the natural way to say it or not. People will not correct you. You'll develop a habit. You'll be understood but you won't be correct. Maybe somebody will finally correct you and then you'll have to stop and think and remember what you were taught so you can fix your bad habit. I believe that's called a language monitor! You'll have to monitor yourself to make sure you don't slip up. ...And so you slip into the downward spiral of mediocrity.
OK, I think I've gone further than what I wanted to say. So let me summarize what the most important thing in this post was. That is, our brain can do all the work of acquiring the language. Given enough volume and variety of input, the brain organizes the language and makes the connections which in turn creates the ability to comprehend the language.
Can we expect this to happen with just 100 or 400 hours of input? Definitely not with only 100 hours. And you certainly cannot finish at the half-way point and come to a valid conclusion.
I do recommend that you be interested in the input. Listening to only audio input that you don't understand would probably not be interesting which, in turn, would make you drift off into thinking about something else, which may or may not affect the effectiveness of the language that you are trying to let into your head. So, if you're not into it, you'll most likely not keep up with it. You'll forget to watch or listen to whatever it is that you were using.
So by all means, find something interesting and if you think that makes it comprehensible for you, that's fine. When I can figure out what is going on in a TV drama story, I find it interesting even though I admit that I am not actually comprehending 99 percent of it. What I do understand about the plot of the story is mostly from my imagination backed up by how I saw the characters interact. I can see that the girl's parents don't approve of her boyfriend and I imagine that somewhere in that dialogue they are telling her she can't see him anymore. I don't know that for a fact but I can stay interested in the story and keep watching and keep listening to the incomprehensible language.
I think I've just come up with a new term. Interesting Input!